1. Field of the Invention
The present invention is applied to angular indexing devices of machine tools, or to a rotary indexing device including an indexing mechanism that indexes an angular position of a circular table by rotating the circular table with the use of a driving motor as a driving source, a clamping mechanism that holds the indexed angular position, and a control unit that controls drive of the driving motor by position control. In particular, the present invention relates to a method of controlling the driving motor during operation of the clamping mechanism of the rotary indexing device.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication No. 10-29125 discloses a rotary indexing device of a machine tool to which the present invention is applied. The rotary indexing device disclosed in the publication has a table (circular table) rotatably supported at a base, and an indexing mechanism that indexes the angular position of the table by rotating the table. A typical indexing mechanism has a worm wheel concentrically fixed at the table, and a worm meshing with the worm wheel and rotated by a rotary drive device (for example, a driving motor, or a servomotor). The publication also discloses that the indexing mechanism employs a direct-drive driving motor composed of a motor rotor fixed at the table, and a motor stator fixed at the base to rotate the motor rotor.
Also, a rotary indexing device like one mentioned above typically has a clamping mechanism that holds the indexed angular position of the circular table. The rotary indexing device disclosed in the publication has a clamp ring as the clamping mechanism. The clamp ring is fixed at the base and has a clamping surface facing the circumferential surface of the circular table. The clamp ring is deformed in a diameter-contraction direction because of the action of pressure fluid or the like, so that a clamping force of the clamp ring inhibits the circular table from rotating relative to the clamp ring, so as to hold the angular position of the circular table. In many cases, the angular position of the circular table is held when a workpiece mounted on the circular table is processed.
When the workpiece is processed, a large load may be applied to the circular table through the workpiece. The load may act as a force in a direction to rotate the circular table. As a result, the circular table may rotate although the clamping force of the clamping mechanism acts on the circular table. Due to this, the indexed angular position of the circular table may be deviated. Such a deviation of the angular position may be also caused by torsion of a driving system.
The deviated angular position of the circular table may cause the rotational position (phase) of the driving motor as the driving source for the indexing mechanism to be deviated from a target rotational position. In the rotary indexing device in which the drive of the driving motor is controlled by the position control (servo control), when a deviation is generated at the rotational position of the driving motor as described above, the position control performs control for restoring the deviated angular position of the circular table to a correct position. Also, regarding the related art, the position control during clamping is performed with the same content of control as that during indexing the angular position of the circular table (hereinafter, referred to as “angular indexing”).
However, since the clamping force of the clamping mechanism acts on the indexing mechanism during the processing of the workpiece, the clamping force may act on the driving motor as a load when the indexing mechanism is driven. Accordingly, if the position control for restoring the deviated angular position to the correct position with the same content of control as that for angular indexing with a high responsibility, the driving motor may become an overload state. When the overload state of the driving motor is detected, the state is determined as an abnormal state and a stop signal is generated. With the stop signal, the rotary indexing device and the machine tool may be stopped and the processing of the workpiece may be interrupted. The processing of the workpiece may be frequently interrupted, which degrades the efficiency of processing.
The above-described problem may occur not only when the angular position of the circular table is deviated during processing of the workpiece (in a clamping state), but also depending on the operation timing of the clamping mechanism during angular indexing. In particular, when the driving motor is rotated by the servo control to the target angular position so as to index the angular position of the circular table, the operation timing of the clamping mechanism does not correspond to a timing in which the positional deviation becomes zero, and typically has a certain tolerance concerning an imposition width. Accordingly, a condition may be conceived that the clamping force acts on the indexing mechanism before the positional deviation becomes zero. Such a condition may cause the overload state as described above.
To prevent the interruption of the processing of the workpiece, the position control of the driving motor may be turned OFF during clamping of the circular table. In such a case, even if the angular position is deviated from a desired angular position, the processing may be continued with the deviated angular position, thereby causing a processing failure of the workpiece.